Images taken with a digital camera are often blurred due to imperfect focus, camera motion, and in-camera optical and sensor characteristics (e.g., raw to color conversion for single chips). As a result these images are often post-processed to increase sharpness. This accounts for the many commercially available digital image processing software applications that include an image sharpening feature. In addition, many digital cameras now include in-camera sharpening as part of a post-processing stage before the images are output.
Typically, these sharpening operations involve the use of some form of high-pass filter. Unfortunately, one of the major drawbacks associated with sharpening is its tendency to produce a halo effect (which is sometimes called ringing). The halo effect is characterized by bright regions of the image that are adjacent to dark regions becoming brighter than their surrounding pixels. In addition, dark regions of the image that are adjacent to bright regions get darker than their surrounding pixels.